1. Field of the Invention
The present invention is directed to coating compositions based on blocked polyisocyanates and aromatic polyamines which have good storage stability.
2. Description of the Prior Art
Coating compositions based on a blocked polyisocyanate component and a component containing isocyanate-reactive hydrogens are known. The purpose of the blocking agent is to prevent the polyisocyanate from reacting with the isocyanate-reactive component at ambient temperature conditions and thus allow the two components to be mixed and stored prior to their actual use. When the composition is baked at an elevated temperature, the blocking agent is released and the reaction of the two components commences.
It is desirable to use blocking agents for the polyisocyanate which can be released at low curing temperatures in order to reduce energy costs. U.S. Pat. Nos. 2,801,990; 3,779,794; 4,007,215; 4,087,392; 4,101,530; 4,132,843; and 4,332,965; British Pat. Nos. 1,442,024 and 1,523,103; German Offenlegungsschrift No. 2,623,081 and German Auslegeschrift No. 2,639,491 described polyisocyanates blocked with C-H acidic compounds which can be reacted at lower temperatures than polyisocyanates blocked with other known blocking agents. The disadvantage of compositions based on polyisocyanates blocked with C-H acidic compounds and either aliphatic amine or hydroxyl co-reactants is that they must contain stabilizers in order to provide sufficient room temperature stability. Note U.S. Pat. Nos. 4,439,593 and 4,518,522. To the contrary, polyisocyanates blocked with oximes such as methylethylketoxime have better storage stability with hydroxyl co-reactants, but require higher curing temperatures.
Accordingly, it is an object of the present invention to provide coating compositions which cure at low temperatures and have improved storage stability, especially when compared to blocking agents which require higher curing temperatures. It was surprisingly found that these objects could be achieved in accordance with the present invention as described hereinafter.